Pointers

1. Programming Pointers

2. Pointers • A pointer is a variable used for storing the address of a memory cell. • We can use the pointer to reference this memory cell Memory address: 1020 1024 1032 … … 100 … 1024 … integer pointer

3. Computer Memory • A variable is in fact a portion of memory to store a determined value • Each variable is assigned a memory slot (the size depends on the data type) and the variable’s data is stored there Memory address: 1020 1024 1032 … … 100 … 1024 … a Variable a’s value, i.e., 100, is stored at memory location 1024 int a = 100;

4. Pointer Types • Pointer • C++ has pointer types for each type of object • Pointers to int objects • Pointers to char objects • Pointers to user-defined objects (e.g., RationalNumber) • Even pointers to pointers • Pointers to pointers to int objects

5. Address Operator & • The "address of " operator (&) gives the memory address of the variable • Usage: &variable_name Memory address: 1020 1024 … … 100 … … … a int a = 100; //To get the value, use the variable name cout << a; //prints 100 //To get the memory address, add the address //operator before the variable name cout << &a; //prints 1024

6. Memory address: 1020 1024 1032 … 88 100 … … … a b Address Operator & #include <iostream> using namespace std; void main(){ int a, b; a = 88; b = 100; cout << "The address of a is: " << &a << endl; cout << "The address of b is: " << &b << endl; } Result is: The address of a is: 1020 The address of b is: 1024

7. Pointer Variable • A pointer variable is a specific box for storing a memory address • Declaration of Pointer variables type* pointer_name; //or type *pointer_name; Where type is the type of data pointed to (e.g. int, char, double)

8. Memory address: 1020 1024 1032 … 88 100 … 1024 … a p int a = 100; int *p = &a; cout << a << " " << &a <<endl; cout << p << " " << &p <<endl; Result is: 100 1024 1024 1032 Pointer Variables • The value of pointer p is the address of variable a • A pointer is also a variable, so it has its own memory address

9. Reference Operator * • We can access to the value stored in the variable pointed to by preceding the pointer with the “star” reference operator (*), Memoryaddress: 1020 1024 1032 … 88 100 … 1024 … a p int a = 100; int *p = &a; cout << a << endl; cout << &a << endl; cout << p << " " << *p << endl; cout << &p << endl; Result is: 100 1024 1024 100 1032

10. Don’t get confused • Declaring a pointer means only that it is a pointer: int *p; • Don’t be confused with the reference operator, which is also written with an asterisk (*). They are simply two different tasks represented with the same sign int a = 100, b = 88, c = 8; int *p1 = &a, *p2, *p3 = &c; p2 = &b; // p2 points to b p2 = p1; // p2 points to a b = *p3; //assign c to b *p2 = *p3; //assign c to a cout << a << b << c; • Result is: 888

11. Pointer Example Result is value1==10 / value2==20 #include <iostream> using namespace std; int main (){ int value1 = 5, value2 = 15; int *p1, *p2; p1 = &value1; // p1 = address of value1 p2 = &value2; // p2 = address of value2 *p1 = 10; // value pointed to by p1=10 *p2 = *p1; // value pointed to by p2= value // pointed to by p1 p1 = p2; // p1 = p2 (pointer value copied) *p1 = 20; // value pointed to by p1 = 20 cout << "value1==" << value1 << "/ value2==" << value2; return 0; }

12. Another Pointer Example int a = 3; char s = ‘z’; double d = 1.03; int *pa = &a; char *ps = &s; double *pd = &d; cout << sizeof(pa) << sizeof(*pa) << sizeof(&pa) << endl; cout << sizeof(ps) << sizeof(*ps) << sizeof(&ps) << endl; cout << sizeof(pd) << sizeof(*pd) << sizeof(&pd) << endl;

13. Traditional Pointer Usage void IndirectSwap(char *Ptr1, char *Ptr2){ char temp = *Ptr1; *Ptr1 = *Ptr2; *Ptr2 = temp; } int main() { char a = 'y'; char b = 'n'; IndirectSwap(&a, &b); cout << a << b << endl; return 0; }

14. Pass by Reference void IndirectSwap(char& y, char& z) { char temp = y; y = z; z = temp; } int main() { char a = 'y'; char b = 'n'; IndirectSwap(a, b); cout << a << b << endl; return 0; }

15. Alternative Pass by Reference void IndirectSwap(char &y, char &z) { char temp = y; y = z; z = temp; } int main() { char a = 'y'; char b = 'n'; IndirectSwap(a, b); cout << a << b << endl; return 0; }

16. Pointer to Pointer What is the output? 58 58 58

17. More Pointer to Pointer p q r s a 58 345670 445670 545670 645670 integer a; Pointer to integer Pointer to Pointer to integer Pointer to Pointer to pointer to integer Pointer to Pointer to pointer to Pointer to integer int a = 58; int *p = &a; int **q = &p; int ***r = &q; int ****s = &r; q = &a //illegal! s = &q //illegal!

18. Pointers and Arrays • The name of an array points only to the first element not the whole array. 1000 1004 1008 1012 1016

19. Array Name is a pointer constant #include <iostream> using namespace std; void main (){ // Demonstrate array name is a pointer constant int a[5]; cout << "Address of a[0]: " << &a[0] << endl << "Name as pointer: " << a << endl; } /* result: Address of a[0]: 0x0065FDE4 Name as pointer: 0x0065FDE4 */

20. Dereference of An Array Name This element is called a[0] or *a #include <iostream> using namespace std; void main(){ int a[5] = {2,4,6,8,22}; cout << *a << " " << a[0] << " " << *(&a[0]); } //main 2 2 2 a[0] 2 a a[1] 4 a[2] 6 8 a[3] a[4] 22 a

21. p a a[0] 2 a[1] 4 6 a[2] a[3] 8 a[4] 22 2 2 Array Names as Pointers • To access an array, any pointer to the first element can be used instead of the name of the array. We could replace*p by *a #include <iostream> using namespace std; void main(){ int a[5] = {2,4,6,8,22}; int *p = a; int i = 0; cout << a[i] << " " << *p; } a

22. Multiple Array Pointers • Both a and p are pointers to the same array. #include <iostream> using namespace std; void main(){ int a[5] = {2,4,6,8,22}; int *p = &a[1]; cout << a[0] << " " << p[-1]; cout << a[1] << " " << p[0]; } A[0] 2 2 4 4 a[0] 2 p a[1] 4 a[2] 6 P[0] a[3] 8 a[4] 22

23. 2 4 6 8 22 Pointer Arithmetic • Given a pointer p, p+n refers to the element that is offset from p by n positions. p - 1 a p a + 1 p + 1 a + 2 p + 2 a + 3 p + 3 a + 4

24. 2 a 4 a + 1 6 a + 2 8 a + 3 22 a + 4 Dereferencing Array Pointers a[0] or *(a + 0) a[1] or *(a + 1) a[2] or *(a + 2) a[3] or *(a + 3) a[4] or *(a + 4) *(a+n) is identical to a[n]

25. int a = 1, b = 2, c = 3; int *p[5]; p[0] = &a; p[1] = &b; p[2] = &c; int list[5] = {9, 8, 7, 6, 5}; int *p; P = list;//points to 1st entry P = &list[0];//points to 1st entry P = &list[1];//points to 2nd entry P = list + 1; //points to 2nd entry Array of Pointers & Pointers to Array a p b c A pointer to an array An array of Pointers

26. NULL pointer • NULL is a special value that indicates an empty pointer • If you try to access a NULL pointer, you will get an error int *p; p = 0; cout << p << endl; //prints 0 cout << &p << endl;//prints address of p cout << *p << endl;//Error!

27. Storing 2D Array in 1D Array int twod[3][4] = {{0,1,2,3}, {4,5,6,7}, {8,9,10,11}}; int oned[12]; for(int i=0; i<3; i++){ for(int j=0; j<4 ; j++) oned[i*4+j] = twod[i][j];}

28. table[ 0] or *( table + 0 ) table[ 2] or *( table + 2 ) table[ 1] or *( table + 1 ) Pointer to 2-Dimensional Arrays table table + 1 table + 2 int table[3][4] = {{1,2,3,4}, {5,6,7,8},{9,10,11,12}}; table[i][j] What is **table ? for(int i=0; i<3; i++){ for(int j=0; j<4; j++) cout << *(*(table+i)+j); cout << endl; }